Method of manufacturing thin-walled tubes

ABSTRACT

A NEW METHOD OF MAKING THIN-WALLED TUBES ACCORDING TO WHICH A TUBULAR BLANK IS ROLLED IN SUCH A MANNER AS TO FORM A SHORT SECTION OF RELATIVELY LARGE DIAMETER AT EACH END, EACH FOLLOWED, AS THE CENTER OF THE TUBE IS APPROACHED FIRST BY A NECK, AND THEN BY A SEGMENT OF GRADUALLY INCREASING DIAMETER. THE END SECTIONS OF LARGE DIAMETER ARE THEN CUT OFF AND THE REMAINDER OF THE BLANK ROLLED TO THE DESIRED THICKNESS IN A REDUCING MILL.

Feb. 9, 1971 J. J. VERDICKT METHOD OF MANUFACTURING THIN-WALLED TUBES FiledMay 16. 1968 United States Patent 3,561,246 METHOD OF MANUIi AUCTISIRING THIN-WALLED Jacques Joseph Verdickt, Malo-les-Bains, France, assignor to Societe Anonyme dite: Vallourec Usines a tubes de Lorraine-Escaut et Vallourec Reunies, Paris, France Filed May 16, 1968, Ser. No. 729,790 Claims priority, application France, May 18, 1967, 106,854 Int. Cl. B21b 23/00 US. Cl. 72--365 4 Claims ABSTRACT OF THE DISCLOSURE A new method of making thin-walled tubes according to which a tubular blank is rolled in such a manner as to form a short section of relatively large diameter at each end, each followed, as the center of the tube is approached first by a neck, and then by a segment of gradually increasing diameter. The end sections of large diameter are then cut off and the remainder of the blank rolled to the desired thickness in a reducing mill.

This invention relates to a new method of manufacturing thin tubes.

This method makes use of a particular type of a rotary tube rolling mill described in French Pat. No. 1,475,645 characterized by the fact that it comprises several rolls distributed about the periphery of the tube to be rolled, each of which rolls is carried in a swivel-mounted roller bearing in the housing of the machine, and a roller bearing mounted in a movable end bracket, the position of which may be angularly adjusted relative to that of the housing.

It follows that, by changing the angular position of this bracket relative to the housing, both the inclination of the axes of the rolls relative to the axis of the tube being rolled, and the diameter to which the tube is being reduced are simultaneously regulated.

It is an object of the present invention to provide a new method of manufacturing thin tubes characterized by the fact that a tubular blank is rolled by controlling the angular position of the movable bracket relative to the housing of the roll so as to produce, after rolling, a product the thickness of which decreases abruptly near each end, and then increases gradually for a certain distance toward the center until it attains a normal thickness which is maintained over the major portion of the tube. The ends of the blank may then be cut off, after which its thickness is further reduced by passing it through a reducing mill comprising several stands of rolls driven at increasing speeds so as to simultaneously reduce the section and increase the length of the tubular blank.

In a particular method of carrying out the invention, the angular movement of the movable bracket is so controlled as to produce only a slight reduction in the thick ness of the workpiece near the rear thereof between the end which passes through the rolls last and the zone adjacent said end which has the smallest diameter after rolling has been completed. By following this procedure it is possible to substantially reduce the thickness of a tubular blank without causing, at the end of the rolling, excessive deformations which, by forming a lobular section, for example, may jam the rolls.

In the present state of the art, a tube is passed through a reducing mill comprising several stands of rolls which successively elongate the tube, so that the ends of this tube, after reduction, have an excessive thickness which results from the fact that the reducing mill is incapable Patented Feb. 9, 1971 of applying sufiicient tension to said ends to cause a reduction in their thickness.

The combination according to the invention of a rolling mill of the type described in the second and third paragraphs of this specification, and a reducing mill, makes it possible to produce tubes of constant thickness in an extremely simple and economical manner, without having to cut off and reject a substantial length at each end of the tubes, as is the case in most methods now being used.

It is a further object of the present invention to provide, as a new article of manufacture, a rolled tubular blank adapted to have its section reduced in a reducing mill having a plurality of stands of reducing rolls, which blank is essentially characterized by the fact that it has at each end a zone in which the thickness diminishes as that end is approached, until, at the extreme end, the thickness increases rapidly until it becomes as great or greater than the thickness of the central part of the blank.

In order that the invention may be better understood, a preferred embodiment thereof will now be described, purely by way of example, with reference to the accompanying drawing.

The single figure of the drawing schematically represents an axial section through a rolled blank according to the invention which is adapted to be introduced into a mill comprising a plurality of stands of reducing rolls.

For convenience in preparing the drawing, the central part of the blank has been broken away, and the variations in thickness have been exaggerated.

The blank shown on the drawing has been shaped by passing the end shown at the left of the figure through the mill first so that the end at the right passed through last.

The rolling mill used is one which comprises, for example, three rolls regularly distributed about the periphery of the tube. Each roll, which is rotatably driven, is delimited by a surface of revolution and comprises a circular shoulder, which, in cooperation with a rotating mandrel inside the blank, at the right of the rolls, causes the reduction in the thickness of the blank, while maintaining its inner diameter uniform.

In order to carry out the process according to the invention, the rolling mill is provided with automatic control means by which the angular position of the movable support with respect to the housing may be varied in dependence on the length of the blank which has already been rolled.

This apparatus may, for example, comprise pneumatic or hydraulic cylinders controlled by photo-electric cells or the like, which detect the passage of one end or the other of the blank.

In the method of operation described, the rolling mill reduces the thickness of the wall of the blank to a substantial extent such that if its thickness were reduced to that extent throughout its length, the trailing end of the blank would be deformed to an extent sufficient to cause jamming of the rolls, or comparable operational mishaps.

For this reason, in accordance with a preferred method of carrying out the invention, the trailing end 2 of the blank has a thickness E which is substantially greater than the thickness e of the leading end of the blank.

In accordance with the invention the blank decreases sharply in thickness near its leading end to form a sort of shoulder 3, from which the thickness increases progressively over a section L until it reaches the normal thickness of the central part 4.

In the embodiment illustrated in the drawing, the thickness of the central part 4 of the blank is equal to the thickness e of the leading end 1, but the two thicknesses could differ.

There is also an abrupt decrease in thickness near the trailing end 2 of the blank, which results in a shoulder 5, and the thickness of the blank thereafter increases progressively over a section L between said shoulder and the central part of the blank.

The thicknesses of the blank at the necks adjacent the shoulders 3 and 5 as well as over the sections L and L depend on the characteristics of the reducing mill which is to reduce the thickness of the blank.

These dimensional characteristics of the blank may be determined experimentally. However, to a first approximation, the volumes of revolution by which the ends of the blank are reduced correspond substantially to the increases in volume resulting from the increase in the thickness of the finished tube which would be produced by passing a cylindrical blank of constant thickness having the same characteristics as the central part 4 of the blank 4 through the reducing mill.

In a particular method of carrying out the invention, the process starts with a blank of ordinary carbon steel having an external diameter of 80 mm. and a thickness of mm. This blank is rolled in a mill of the type hereinbefore indicated so as to produce a blank having the characteristics shown on the drawing.

The thickness 2 of the central part 4 and the leading end 1 of the blank are, after rolling, reduced to 2.8 mm., whereas the thickness E of the trailing edge 2 is only reduced to 4 mm. The thickness just beyond the bases of the shoulders 3 and 5 is 2.55 mm. and the sections L and L are each 500 mm. long.

During the rolling a mandrel 47 mm. in diameter is kept inside the blank at the right of the rolls.

After cutting off the ends 1 and 2 of the resulting blank, this blank is introduced into a reducing mill comprising 24 stands of annular reducing rolls, the diameter of the gap between the rolls of the first stand being 67 mm., whereas the diameter of the gap between the rolls of the last stand is 25 mm. The distance between the first and last stands is 4,800 mm. The intermediate stands are regularly spaced over this distance.

The first stand is driven at 0.6 meter/ second and the last at 1.5 m./s.

After passing the blank through this reducing mill, the result is a tube having an external diameter of 25 mm. and a thickness of 3 mm. which is constant throughout its length. Only end sections cm. in length need be cut off.

As a consequence of the invention the lengths of the end sections which must be eliminated to insure the maintenance of the tolerances of the tube are greatly reduced.

Thus, in the case of a conventionally rolled cylindrical blank, it is necessary to cut from 60 to 180 cm. from each end of the tube, depending on the tolerances which can be allowed.

Under the same conditions, when working to the same manufacturing tolerances, it is only necessary to out 01f Cir 4 from 30 to 100 cm. when operating in accordance with the invention.

In other words, the process according to the invention makes it possible to reduce the losses at the ends of the tube by about 50%.

In the case of small diameter tubes, this represents a saving of 40 to points per thousand, that is to say a saving of 4 to 6% in metal used. (The term points means the proportion of metal expressed in thousandths, which is lost during manufacture, or in other words, the difference between the weight of metal used at the beginning of manufacture and the Weight at the end thereof, considering the starting amount of metal to represent 1000 units, or points.)

It will, of course, be appreciated that the method and apparatus hereinbefore described have been given purely by way of example and may be modified as to detail without thereby departing from the the basic principles of the invention as defined by the following claims.

What is claimed is:

1. The method of forming a thin-walled tube which comprises the steps of forming at each end of a tubular blank having a central portion in which the wall thickness is uniform a section in which the wall thickness decreases as the adjacent end is approached, and then reducing the average thickness of said wall by rolling said blank in a reducing mill which simultaneously elongates the blank.

2. The method claimed in claim 1 in which the sections of decreasing wall thickness initially terminate short of the ends of the blank, leaving sections of larger diameter at the extreme ends.

3. The method claimed in claim 2 according to which said end sections of larger diameter are cut off before said blank is introduced into said reducing mill.

4. A tubular blank adapted to be rolled into a thinwalled pipe in a reducing mill, said blank comprising a relatively long central section of substantially uniform external diameter and wall thickness positioned between two sections which decrease continuously and progressively in external diameter and wall thickness toward the adjacent ends of the blank, and two end sections of larger external diameter and having a greater wall thickness than the minimum outer diameter and wall thickness of said sections of decreasing diameter and wall thickness.

References Cited UNITED STATES PATENTS 649,497 5/1900 Wales 72370 LOWELL A. LARSON, Primary Examiner US. Cl. X.R. 29--190; 72-367 

